1. Field of the Invention
The present invention relates to skeletal implants (such as dental implants) and more particularly to a method of designing skeletal implants that promote strain-induced bone tissue growth and maintenance over the entire bone contacting surface of the implant.
2. Description of the Prior Art
Skeletal implants have been used for the replacement of articular joints within the body (e.g. total hip arthroplasty), restoration of aesthetics (e.g. bony retention of ear prosthesis) and replacement of missing teeth (e.g. dental implants). One of the primary failure mechanisms for skeletal implants is implant loosening at the implant-to-tissue interface due to non-physiologic loading profiles.
Roughly 125 million individuals in the United States alone are missing some of their teeth. One approach to treating patients missing teeth is to supply them with removable dentures. Dentures have the disadvantage of not adequately loading their supporting bone (such as the mandible for lower dentures and the maxilla for upper dentures). An unloaded supporting bone experiences very little strain. When the supporting bone lacks a minimum level of strain, bone resorption occurs. This results in shrinkage of the supporting bone and can further result in related health and aesthetic problems.
Another approach to treating edentulous or partially edentulous patients is to place endosteal osteointegrated (the integration of bone tissue with the implant) implants in the supporting bone. Osteointegrated endosteal implants are alloplastic materials surgically inserted into a residual bony ridge to serve as prosthodontic foundations. Such implants serve as platforms for prosthetic devices. The introduction of osteointegrated dental implants has given edentulous and partially edentulous patients a more effective means to restore their ability to chew and to improve their appearance. Furthermore, osteointegrated implants functionally load the mandibular (or maxillary) bone into which they are implanted, thereby inducing strain in the bone under normal functional loading. Bone loss and resorption, which commonly occur with dentures, can thereby be minimized or avoided by maintaining a proper loading profile on the bone.
Two subcategories of endosteal implants include plate form implants and root form implants. A plate form implant is characterized by a flat, narrow plate typically placed in a horizontal dimension of the mandibular or maxillary bone. Root form implants are designed to be placed in a vertical column of bone. Root form implants include two types: cylinder-type root form implants, which are non-threaded cylinders pressed into holes drilled into the receiving bone, and screw-type root form implants, having a threaded outer surface which is screwed into a hole drilled into the receiving bone.
The cylinder root form implant may have design features which minimize rotation of the implant in the implanted bone (e.g. holes and grooves) as well as a textured surface, which promotes close bone apposition to the implant. A disadvantage of cylinder root form implants is that they take a long time to set properly, as the patient must wait until the surrounding bone has properly integrated with the implant before functionally loading the implant.
Screw root form implants are held to the surrounding bone by a threaded outer surface. The threaded surface provides initial stabilization of the implant to the surrounding bone and it facilitates macroscopic bone integration. Because they are screwed into the bone, screw root form implants may not require as much time as cylinder root form implants prior to functionally loading.
Current screw and cylinder root form implants have a disadvantage in that bone resorption commonly occurs in the crestal region of the implant due to excessive strain experienced by the bone in this region. Furthermore, other regions of the bone may not experience enough strain and resorption may also occur. Although a certain level of strain must be experienced by the bone to prevent bone resorption, too much strain can also result in bone resorption. If the supporting bone experiences less than 100 microstrain or more than 3000 microstrain, bone resorption will occur. On the other hand, a strain level between about 100 microstrain and 3000 microstrain can actually encourage bone growth.
Many common screw-type implants impart too much strain in some portions of the implant-bone interface (e.g. the crestal regions) and not enough strain in other parts of the interface. This results in non-uniform bone ingrowth and resorption, which further results in implant loosening. Roughly 10% to 15% of all dental implant patients must eventually return to the implantologist for revision surgery due to inadequate bone ingrowth, loosening, or structural failure of the implant. Revision surgery can be significantly more costly than primary surgery. Furthermore, it often leads to increased failure rates due to surgical complications involving decreased quality of available bone, or bacterial smear layers of contamination on the implant once bone loss occurs.
More than 24 cylindrical shaped and blade-shaped endosteal and transosteal implant systems are available on the market today. These devices include those made by NobelPharma USA, Inc. of Nobel Industries in Sweden developer of the Branemark system, an endosseous fixture which is one of the most popular in the U.S. and which has been given full acceptance by the American Dental Association (ADA). Other devices which have received provisional acceptance by the ADA include: Dentsply (previously Core-Vent) root forms, Oratronics blade implant, and Integral cylindrical implants by Calcitek.
Niznick (U.S. Pat. No. 4,431,416) discloses a combination root form implant having an intermediate section with peripheral threads to engage the bone. The lower end of the implant is hollow and has peripheral holes through which bone tissue may grow. The implant receives a denture which transmits bite force to the gum tissue, thereby reducing the transmission of such force to the implant. Because the Niznick device does not physiologically load the implanted bone, it does not provide strain-induced bone growth.
Friedman et al. (U.S. Pat. No. 5,209,659) discloses a dental implant having a cylindrical body portion and a threaded apical portion which does not exceed one-half of the length of the body. The threaded portion has sharp external cutting threads which do not extend beyond the diameter of the cylindrical portion.
Scortecci (U.S. Pat. No. 5,312,256) discloses a screw-type root form implant that employs a fine pitch thread with a plurality of interruptions of the thread, both of which serve to reduce the internal stress in the bone in order to avoid necrosis. Scortecci does not disclose an implant wherein strain is maintained within a predetermined range in order to encourage bone growth and to reduce resorption.
Weiss et al. (U.S. Pat. No. 4,997,383) discloses a blade-type dental implant with substantial planar areas on the front and rear surfaces of the implant which make bone contact produce optimal force absorption in areas of highest stress. Weiss et al., however, does not disclose an implant designed to produce a level of strain in the implanted bone that would promote bone growth.
Valen (U.S. Pat. No. 5,007,835) discloses a screw-type root-form implant having rounded screw threads to provide radial forces at points in contact with the bone. A separate tapping mechanism is also disclosed. Although Valen attempts to reduce bone necrosis by employing rounded threads, Valen does not disclose a means to ensure that strain in the bone surrounding the implant is maintained within a predetermined range.
None of these devices have been developed to stimulate and maintain bone strain levels over the entire surface area of the implant which promote osteointegration while minimizing bone resorption.
Several references disclose methods of maintaining and promoting bone growth by applying mechanical stimuli. Mcleod et al. (U.S. Pat. Nos. 5,103,806 and 5,191,880) describes a method for preventing osteopenia and promoting bone growth by applying a mechanical load to the bone tissue at relatively low magnitudes and at relatively high frequencies. Although these patents suggest that the disclosed methods can be used in conjunction with prosthetic implants, they do not propose a particular implant geometry or a method by which to derive such a geometry.
Lanyon, in Control of Bone Architecture by Functional Load Bearing, 7 Journal of Bone and Mineral Research S369-S375 (Supp. 2, 1992) describes the importance of local functional strains in the control of bone architecture. This article discusses the adaptation and maintenance of bone as being predominantly due to a conservational or "osteogenic" strain regime sustained at each location of concern within the skeletal system.
None of these references disclose a method of designing an implant which optimizes the geometry of the implant to provide the level of strain on the implanted bone required to maximize osteointegration and minimize bone resorption. Nor do any of these references disclose an implant design which provides the level of strain on the implanted bone required to maximize osteointegration and minimize bone resorption over the entire bone contacting surface area of the implant.
Thus, there exists a need for a method of designing an implant so that the implant creates in the implanted bone a level of strain which maximizes bone growth and which minimizes bone resorption over the entire bone contacting surface area of the implant.
There also exists a need for an implant that promotes maintenance of bone under functional loading conditions over the entire bone contacting surface area of the implant.